I am facing somewhat the same issue with peds.
For example, creating a ped out of a specific range ( I believe the 300 units arround the player that are mentioned here) results in the entity to be deleted (tested it with DoesEntityExist in a loop while changing coords).
By adding a blip to the entity you can see just a for a bit that the entities are created and instantly deleted.
Though, entities that get created within the ‘scope’ are persistent.
I have tested it with 2967 build and It’s working fine, but using latest build (3270) or latest recommended (3184), entities in general seem to disappear.
This issue was resolved in 2689 if I understand what am I reading here.
[Edit] To my surprise, this is happening both on infinity and legacy mode. In legacy mode, the ‘scope’ that entities are created and don’t get deleted also is smaller.